The present invention relates generally to the field of ringing controllers, and specifically to ringing controllers for providing switching between battery feed and ringing states.
With increased demand to deliver high-speed data services to subscribers, many techniques have been developed to utilize existing telephone loops to carry data signals simultaneously with normal voice band telephony services. Generally, data signals are carried in a frequency range, referred to as a data band, that is above the voice band. However, signals associated with plain ordinary telephone service (POTS), such as on-hook and off-hook transitions and ringing, generate high noise levels at frequencies above the voice band and, thus, this noise falls in the data band causing interference. Traditionally, large and costly filters, often referred to as “POTS Splitters”, have been employed at the telephone equipment location to remove POTS-created noise from the data band. This requirement has been an impediment to the large-scale deployment of data services.
The generally accepted method of alerting a subscriber that an incoming telephone call has arrived is to apply a high magnitude AC waveform to the subscriber loop in order to ring a bell or similar audible alerting device at the subscriber's premises. One very common standard in North America is to use 86 V rms alternating current signals with a frequency of 20 Hz, although other different voltages and frequencies can be employed. This AC waveform is often referred to as the power-ringing signal.
The process of ringing a subscriber's line can be considered as a transition between two states. A first state is providing a battery feed to the loop, which may include on-hook transmission or supervision, and when the subscriber is off-hook and connected to another subscriber. Second, the state of providing the power-ringing signal to the loop to alert the subscriber that another subscriber is calling.
Data transmission in the data band is provided over the subscriber line at all times during these two states, and during transitions between these two states. However, data signals require very good signal to noise ratios to achieve the high throughputs required by applicable industry standards and are quite susceptible to noise, both in the time and frequency domains. Thus, it is desirable that transitions between the battery feed and power-ringing states cause a minimum amount of noise in the data band.
Traditionally, mechanical relays have been employed to switch power-ringing signals onto the subscriber loop. It is an unfortunate characteristic of relays that they tend to introduce discontinuities onto the loop voltage due to timing variations, abrupt switching behavior, contact bounce, or open-circuit intervals between states. Large voltage discontinuities manifest themselves as high frequency noise, which interfere with data signals. Thus, it is desirable that the change between the battery feed state and the ringing state be continuous and smooth to avoid creating noise artefacts that interfere with transmission in the data band.
More recently, solid-state relays have been employed to switch power-ringing signals. In a co-pending application, the need for a traditional POTS splitter filter is reduced by timing the removal and application of the battery state and the ringing states through monitoring the voltage zero crossing and applying feedback techniques. This approach makes the ringing transitions contribute little interference in the data band. Such an implementation is illustrated in FIG. 1. However, in order for this to operate properly, the switching process may begin prior to the next zero crossing of the power-ringing signal and battery supply, thus prediction and timing of this trigger event should be taken into account. This implies that adaptation for different ringing frequencies would need to be implemented, which may vary from country to country.
Further, the approach described with reference to FIG. 1 is implemented in a “make before break” manner. That is, the battery feed is applied to the subscriber line before the ring source is removed. The disconnection of the ring source from the subscriber line is timed precisely in a short interval, to within 1/20th of the period of the ring source, from the voltage crossing of the ring source and the battery. However, any residual currents in the load, due to inductive elements, will flow into the battery. These currents can be quite large and require a large low impedance device to pass these currents between the battery and the load. Although large discrete devices are available, in certain applications a large device may be expensive in terms of silicon area, and may be difficult and expensive to integrate.
Therefore, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.